Effect of dietary fiber components on fecal nitrogen excretion and protein utilization in growing rats

The effects of purified fiber components and wheat bran on several indices of protein utilization were determined in growing rats. A control diet containing 10% casein was diluted with either cellulose (C), pectin (P), lignin (L), guar gum (G), or wheat bran (W) at fiber levels ranging from 3% to 20%. All fibers except C caused a decrease in net protein ratio (NPR) as compared to the control casein diet. This depression in NPR increased as the dietary fiber level increased. Apparent and true nitrogen digestibilities also decreased with all fibers at all levels. At the highest level of fiber (20%) the depression was greater for G and W and was least for C. NPR when divided by digestibility (analogous to biological value) decreased with P,L, W (all levels) and G (20% level) but not with C. When rats were fed fiber without protein, there was increased excretion of endogenous fecal nitrogen with all fibers at all levels. The results demonstrate that fiber(s) affected protein utilization as measured by NPR, digestibility and endogenous fecal nitrogen excretion and that the negative effect increased with the level of fiber consumed.